By John Rezabek, Contributing Editor
When I queried the Foundation fieldbus-registered product catalog for "final control element" and found 47 products from 25 different manufacturers, I thought, these can't all be positioners. Three to five appear to be re-branded versions of another OEM's positioner, but I was still impressed with the number and diversity. By my count, there are at least 19 distinct "positioners"—as in a traditional valve positioner supporting a continuous 0% to 100% signal—registered. Add to this 10 motor operated valves (MOVs)—which normally support both on-off and "throttling" (0% to 100%) modes, and 12 offerings aimed exclusively at solenoid-operated on-off applications. I never did this query back in 2006, but I have to believe this is a vast improvement and a real positive for end users. I'm willing to bet a comparable diversity of products is available for Profibus PA; all of these OEMs want to accommodate their end-user communities, regardless of their device-level bus preferences.
What should guide us in the selection of a bus technology—if any—when we choose final control elements for on-off applications? MOV actuator manufacturers have offered their own proprietary buses for many years, but they have a few glaring disadvantages. A plant can have a dozen or more actuated valves in a relatively small geographic area, but inevitably a couple end up isolated, that is, not very handy to integrate with the "mothership." If you want them on your digital bus, you have to get the network to them. Designing it can be cumbersome, costly and time-consuming.
Another issue with proprietary buses is the expertise and engineering tools required to maintain them and their interfaces to the ultimate host system. I use a proprietary multiplexer for indicate-only RTD I/O, and I have a little heartburn every time I need to add a point, which might be months or years apart, because it has a unique programming interface and a unique method to make changes. I hope the guys and gals who helped me set it up 12 years ago haven't retired yet, because I will be calling them the next time a point is added. And they have to stay on for the poor schlimazels that have to slog through some long-gone engineer's creation. Maintaining the expertise for seldom-used, unique design tools can become burdensome for the end user. Furthermore, the end user is henceforth locked in to his MOV supplier if he wants his actuators to be digitally integrated.
How about AS-i? There are solutions that will get you into Division 2, although a bona-fide "intrinsically safe" install still may be complicated. Aside from its hazardous-area drawbacks, AS-i also has a 100-meter distance limitation. Profibus DP affords the ability to add an AS-i drop at will, but doing so wherever an on-off valve might appear on the plot plan presents the same problems as proprietary bus solutions. AS-i is purely a "bit bus," so little added intelligence or diagnostics are passed on to the host system.
Profibus PA and Foundation fieldbus (FF)afford the end user the flexibility to integrate on-off valves—whether MOV-, solenoid- or continuous positioner-actuated—wherever the process places them. PA and FF allow them to sit on the same bus with analog devices, which can be much more numerous, depending on the process. There is no need to run a special spur to an isolated actuator, and the OEM is free to offer the user extensive diagnostics through FDT-DTM and/or EDDL.
The fieldbus versions are more expensive than AS-i, and there's a price breakpoint around $1000 where you can find continuous positioners for a small premium. These afford an easy path to partial stroke testing—useful even in non-SIL-rated applications. The "breakaway torque" can become excessive when an on-off valve in a process plant sits motionless for months at a time.
Users and their consultants should consider the lifecycle costs of on-off valves. The premium paid for process bus capabilities may be more than offset by their advantages in the design and operations phases.